1. Field of the Invention
The present invention relates to an apparatus which has an electric control device to control the assisting force produced by a power steering system, more particularly to an apparatus capable of changing the assisting force during returning operations of the steering wheel.
2. Discussion of the Prior Art
Usually, a power steering system for a vehicle is provided with a steering shaft connected to a steering wheel, and the steering shaft is connected to the pinion shaft of the steering mechanism through an elastic member such as a torsion bar. The servo valve of the system is actuated by a relative rotation between the steering shaft and the pinion shaft to control fluid flow to and from the power cylinder, which produces an assisting force. In this system, the elastic member is twisted by a manual torque applied to the steering wheel so that the valve is operated to supply working fluid to the power cylinder. As a result, an assisting force is produced by the cylinder.
Further, such power steering system is provided with an electric control device to change the assisting force in accordance with conditions of the vehicle such as vehicle speed, steering angle, and steering speed. To improve the steering feeling, the assisting force is required to be changed during returning operations of the steering wheel. A power steering system disclosed in the Japanese Patent Laid-Open No. 62-279170 is provided with such control apparatus in which the pressure of the working fluid supplied to the reaction mechanism built in the servo valve is controlled by a solenoid throttle valve. Therefore, reaction force generated by the reaction mechanism is changed in accordance with control current applied to the solenoid throttle valve. A base control current is first determined based upon the vehicle speed and the steering angle. In addition to that, a compensation current is determined using a control map memorizing change of the compensation current with respect to change of the steering angle or the steering speed. Thus, a compensated control current, which is obtained by adding the compensation current to the base control current, is applied to the solenoid throttle valve when the steering wheel is returned to its neutral angle position. On the other hand, the base control current is applied to the solenoid throttle valve as it is when the steering wheel is turned from the neutral angle position to the right or left. With this control, the steering wheel is easily returned to its neutral angle position, improving the feeling of the steering operation.
In the embodiments, a control map defining change of the compensation current with respect to change of the steering angle and a control map defining change of the compensation current with respect to change of the steering speed is selectively used to calculate the compensation current. Therefore, the apparatus cannot change the assisting force during the returning operations in response to both of the steering angle change and the steering speed change. In the first embodiment, when the steering angle is large, the compensation current always becomes large regardless of the steering speed. In the second embodiment, when the steering speed is large, the compensation current always becomes large regardless of the steering angle. Therefore, it is difficult to give the driver a good steering feeling.
Further, in the prior apparatus, the compensation current is not determined based upon the amplitude of the base current, but determined base upon the steering angle or the steering speed. Therefore, even if the amplitude of the compensation current is the same, the amplitude ratio of the compensation current to the base current changes, causing a following disadvantage. Supposing that the base current is 0.8 ampere and the compensation current is 0.2 ampere, a control current applied to the solenoid valve is increased by 25% from 0.8 ampere to 1.0 ampere when the driver starts to return the steering wheel toward the neutral angle position. The reaction force produced by the reaction mechanism increases in accordance with the increase of the control current. On the other hand, in the event that the base current is 0.4 ampere and the compensation current is 0.2 ampere, the control current applied to the solenoid valve is increased by 50% from 0.4 ampere to 0.6 ampere, when the driver starts to return the steering wheel toward the neutral angle position. As explained above, the increase ratio of the control current is twice in the latter case as compared with the former case, even if the amplitude of the compensation current is the same. If the increase ratio becomes too high, the driver has an incongruous feeling. Therefore, it is desirable to suppress the increase ratio within a predetermined range.
Moreover, the prior apparatus, the reaction force is always increased during returning operations of the steering wheel, even if the vehicle is in conditions that the reaction force is required not to be increased. For example, the reaction force must not be increased in the event that the steering wheel is rapidly returned by the driver to avoid a danger. Therefore, the prior apparatus cannot offer appropriate control in such event. The reaction force is usually required to be increased when the vehicle is being accelerated.